Television and like transmitting system



l DeC- 12, 1939- A. D. BLUMLEIN ETAI. 2,182,839

TELEVISION AND LIKE TRANSMITTING SYSTEM4 Filed April 17, 193e 2 sheets-sheet 1 Dec. 12,1939. A D BLUMLElN Er AL 2,182,839

TELEVISION AND LIKE TRANSMITTING SYSTEM Filed April 17, 1936 2 Sheeats-Sheetl 2 Patented Dec. 12, 1939 Nrrs ` 'rELnvIsro-N man newer :simlein AND' LIKE TRANSMITTING SYSTEM and Eric Arthur Nind,

Ealing, London, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application April 17,1936, Serial No. 74,928 In Great Britain April 3, 1935 10 Claims.

The present invention relates to television and like transmission systems of the kind in which synchronising impulses are transmitted in the intervals between trains of picture signals, the

transmission being by modulated carrier wave.

In such systems it is known to be desirable to arrange that the carrier shall be reduced to Zero at the peaks of the synchronising impulses; this is, however, often diflicult of achievement, because of the curvature of the bottom bend of the characteristic of most valves used in the modulated amplier stage, the curvature necessitating what may be an undesirably large amplitude oi synchronising pulse to reduce the anode current of the modulated amplifier valve, and hence the transmitted carrier, to Zero.

The present invention has for its object to provide means whereby the difficulties mentioned above can be overcome.

The present invention provides a method of transmission by modulated carrier, for use in television vand like systems which methodl comprises generating` a carrier oscillation, modulating the amplitude of said carrier in a modulated amplier by trains of picture signals having intervals between trains, and modulating the amplitude of said carrier b-y synchronising signals at a secondary modulating point which is between the carrier oscillation generator and the modulated amplifier, modulation at the secondary modulating point being effected in such a manner that the amplitude of the transmitted modulated carrier is equal substantially to zero during said intervals.

The picture signals may modulate the carrier in such a sense that an increase in picture brightness corresponds to an increase in carrier amplitude, and in synchronising signals may also be fed to the modulated amplifier in such a sense as to tend to reduce the carrier amplitude during the intervals between trains of picture signals.

The invention further provides apparatus for carrying the methods of transmission accord-` ing to the invention into eiect.

The invention will be described, by way of example, with reference to the accompanying drawings, in which Figs. l, Zand 3 illustrate three methods of carrying the invention into eect, I

In Fig. 1 picture signals, which are arranged to contain their direct current component, are applied to the grid of a sub-modulator valve I. These signals are in the form of trains of signals representative of picture values, interspersed with interval signals corresponding to picture black. The interval signals occur in the inter- A vals between the scanning of successive lines and successive complete frames of the object. Syn-' chronising signals are preferably, but not essentially, superimposed upon these interval signals in the blacker-than-black sense, and the picture signals are arranged to be in such sensethat an increase in brightness makes the control grid of valve l more positive.

The anode of the valve I is coupled through a direct-current coupling comprising a suitable bias battery to the grid of a modulator valve 2, the anode of which is connected to the centre point of a transformer 3 and through ar resistance 8 to earth.

A carrier oscillation is applied at terminal 4 to the input of an amplifier 5 which may comprise any desired number of stages. The output of the amplifier 5 is fed to a modulated amplier stage 6, the output of which is taken from the modulated-carrier output terminal 7. The potential differences developed across resistance 8 by the picture signals from the modulator 2 serve to modulate the carrier in stage 6 so that the output at terminal I is in the form of a carrier wave modulated with the picture signals, an increase in picture brightness corresponding to an increase in carrier amplitude.

The synchronising signal amplitude from the modulating tube 2 resulting from the synchronizing signals superimposed upon the interval signals as pointed out above, is, however, assumed to be insuflicient to reduce the carrier amplitude to zero. f

synchronising impulses are applied to the grid of a'valve'9 insuch a sense as to make the grid less negative. The anode of the valve 9 is connected through a condenser I0 to the centre point of the secondary winding of a transformer I I arranged at the input side of amplifier 5, and through a resistance I2 to earth. The synchronising pulses are arranged to make the upper end of resistance I2, and hence the grids of the first valves of amplifier 5, so negative that the anode current in these valves is reduced to zero. It will be evidentV that it can be arranged that the amplitude of the pulses necessary to achieve cut-o at the rst stage is muchless than the amplitude which would be necessary at the resistance t. Thus, irrespective of whether the synchronising signals are present in the signals applied across resistance 8, thecarrer amplitude is reduced to zero by the potential difference developed across resistance I2.

The channel extending between transformers II and 3 can without dinculty be made capable of passing the side band width necessary to represent the synchronising signals.

If desired, where the synchronising pulses have been mixed with the picture signals at a point preceding valve 9, the synchronising pulses applied to the valve 9 may be obtained by separating them from the picture signals by an amplitude selection. Such an amplitude selection may be so effected by so biasing the valve 9 that anode current flows only during the synchronising pulses, the synchronising signals being applied to valve 9 in such sense that they make the grid of this valve less negative.

The slight phase difference which may occur between the modulation points represented by the upper ends of resistance 8 and I2 has not been found noticeable in practice. It will be appreciated that since the synchronizing signals supplied to the tube 9 and the tube I being derived from the saine source as is usual in the art by travelling over different electrical paths, will naturally exhibit a slight difference in arrival time at the output 'I for example, which represents, of course, a phase delay between those synchronizing signals transmitted through the tube and those transmitted together with the picture signals through tube I. However, any slight delay which may occur between the two .modulation points may be compensated for if desired by delaying, with the aid of a suitable delay network SI, the signals established by modulator 2 at the upper end of resistance 8 relatively to the signals fed by valve 9 to the upper end of resistance I2.

Instead of modulating the carrier at the upper end of resistance I2 with synchronsing signals, these signals may be arranged to control the operation of one or more absorption valves upon the circuit between input terminal 4 and transformer 3. The absorption valve or valves may be arranged effectively in parallel with a part of this circuit, such as the primary winding of the output transformer from the amplier 5, and may be normally insulating, the synchronising signals being arranged to render the absorption valve or valves conducting.

An example of a circuit of this kind is given in 2, in which like parts have the same reference as in Fig. l. Trains of picture signals, having synchronising signals in the blacker-thanblack sense occurring in the intervals between trains, are applied to terminals I3 and ILE, the polarity of the synchronising signals being opposite to that of the picture signals. The required phase opposition can be obtained with the aid of a reversing valve or otherwise.

The signals are thus applied to two valves 2 and I5 arranged in push pull, the control grids f which are biased negatively by means of battery-2l. One of these valves, namely, the valve E, acts as in the arrangement of Fig. 1 as the modulator valve, whilst valve I has its anode connected to the control grid of each of two absorption valves I6 and I6', each of these valves being effectively in parallel with different halves of the primary winding of they transformer 3, the centre point of which winding is earthed.

The synchronising pulses are applied in such sense that each synchronising pulse makes the anode of valve I5 less negative, and the absorption valves IE and IG', which are so biased as to be insulating during the trains of picture signals, are thus rendered conducting, the carrier input to the modulated amplifier 6 being thereby reduced substantially to zero. An acceptor circuit I1 offering a low impedance to the carrier frequency is arranged between the grid and the cathode of the valve I6. It will be appreciated that during the time that the absorption valve I@ and I6', are conducting, they act effectively as a short circuit across the primary of the transformer 3 and since the tubes are conducting dui'- ing the synchronising interval, it will be clear that no carrier wave energy can be passed through to the secondary of the transformer 3, and consequently, complete carrier suppression is provided during the interval of the synchonising impulses. The serially connected inductances and condenser I7 insures that the current path during the time that the tubes I6 and I8 are conducting shall be a minimum value in order that the lowest possible impedance across the transformer be provided during the synchronising intervals.

The arrangement of the valves 2 and I5 in push-pull has a number of advantages. For example, cross-neutralisation to remove the effects of inter-electrode capacity can be employed, the neutralising condensers I8 and I9 being provided for this purpose. Furthermore, any resistance in the anode current battery 20 is ineffective in modifying the wave form of the applied signals because if the balance of the push-pull arrangement is perfectly adjusted, the current from the battery 2U is constant.

The full-Wave absorption circuit comprising valvesli and IG may in some cases be replaced by a half-wave arrangement, as indicated in Fig. 3, of the drawings in which parts which appear in Fig. 2 of the accompanying drawings have the same references. In Fig. 3 there is provided a transformer 3, the primary of which is fed with carrier frequency energy through the terminal Il. Picture signals, together with synchronising signals of the opposite polarity are injected at the terminals I3, I4. During the period that the synchronising impulses are effective to render the tube I5 non-conducting, the tube i3 is blocked so that across the primary of the transformer 3 there is eectively no load connected thereto. Under these circumstances, the carrier wave energy is passed on to the secondary through the modulator stage 6 onto the output through the terminal 'L However, in the presence of synchronising signals, the tube I5 is rendered non-conducting and the grid of the tube I6 being connected in the anode circuit, therefore, becomes less negative to such an extent that the tubeli becomes conducting and affords through the acceptor circuit I'l back to ground substantially a short-circuit across the primary of the transformer 3, under which conditions on carrier wave energy is passed on to the secondary of the transformer 3 so that at the output terminal 'i' there is complete suppression of the carrier.

The invention is not limited to the arrangements described and many modifications within the scope of the appended claims will occur to those versed in the art.

We claim:

l. A method of transmission by modulated carrier, for use in television and like systems, which method comprises generating a carrier oscillation, modulating the amplitude of said carrier by trains of picture signals having intervals between trains, and reducing the amplitude of said carrier to zero by modulating by synchronising signals at a secondary modulating point intermediate the generation and modulation of the (ill carrier oscillation by the trains of picture signals, modulation at the secondary modulating point being effected in such a manner that the amplitude of the modulated carrier oscillations is equal substantially to zero during said intervals.

2. A method according to claim 1, wherein said picture signals are arranged to modulate said carrier in such a sense that an increase in picture brightness corresponds toan increase in carrier amplitude.

3. A method of transmission by modulated carrierl for use in television and like systems which method comprises generating a carrier oscillation, modulating the amplitude of said carrier'by trains of picture signals having intervals between trains, and modulating the amplitude of said carrier by synchronising signals at a secondary modulating point intermediate'the generation and modulation of the carrier oscillation by the trains of picture signals, regulating the picture signals to increase the amplitude of the carrier oscillation in direct proportion to the amplitude of the picture signals, and regulating the modulation by the synchronizing signals to reduce the amplitude of the carrier oscillation to zero.

4. A transmission apparatus for use in television and like systems employing a modulated carrier comprising a source of carrier oscillations, a modulated amplier valve adapted to be fed with said carrier oscillations and with trains of picture signals having intervals between them, and secondary modulating means for periodically reducing the carrier oscillations to zero by modulating the carrier oscillations by synchronizing signals.

5. Apparatus according to claim 4, wherein said secondary modulating meansl comprise a secondary modulator valve arranged betweensaid source and said primary modulator valve and adapted to be fed with synchronising signals.

6. Apparatus accordingl to claim 4, wherein said secondary modulating means comprise an absorption circuit adapted to be fed with synchronising signals and to prevent substantially the passage of carrier oscillations to said modulated amplifier valve during said intervals.

7. Apparatus according to claim 4 wherein said secondary modulating means comprise an absorption circuit adapted to be fed with synchronising signals and substantially to prevent the passage of carrier oscillations to said modulated amplier valve during said intervals, said absorption circuit comprising a thermionic valve and a source of biasing potential connected t the valve to maintain the valve at cut ofi potential in the absence of synchronizing signals.

signals from said composite signal and for feeding said synchronising signals to said secondary modulating means.

9. A transmission apparatus for use in television and like systems employing a modulated carrier comprising a source of carrier oscillations,

a modulated amplifier valve adapted to be fed` with said carrier oscillations and with trains of picture signals having intervals between them secondary modulating means comprising an absorption circuit adapted to be fed with synchronising signals and substantially to prevent the passage of carrier oscillations to said modulated amplifier valve during said intervals,v said absorption circuit including a therrnonic valve arranged to be substantially insulating except when synchronising signals are applied to it, for ensuring that the amplitude of the carrier oscillations applied to said modulated amplifier valve during said intervals is substantially zero, a source of composite signals comprising trains of picture signals interspersed with synchronising signals, means for feeding said composite signal to said modulated amplifier valve, means for extracting synchronising signals from said composite signalk and for feeding said synchronising signals to said secondary modulating means, and two valves arranged in push-pull relationship and adapted to have said composite signal applied in antiphase to their control grids respectively,

one of said valves being arranged to feed said composite signal to said modulated amplifier valve, and the other being arranged to feed synchronising impulses to the valve which forms part of said absorption circuit.

ALAN DOWER BLUmiEIN. ERIC ARTHUR NIND. 

